﻿// Copyright (c) Argo Zhang (argo@live.ca). All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
// Website: https://github.com/LongbowExtensions/

namespace Longbow.Modbus.Algorithm;

/// <summary>
/// Modbus CRC-16 查表算法
/// </summary>
public static class ModbusCrc16
{
    // 预计算的CRC表（256个条目）
    private static readonly ushort[] CrcTable =
    [
        0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
        0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
        0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
        0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
        0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
        0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
        0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
        0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
        0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
        0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
        0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
        0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
        0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
        0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
        0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
        0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
        0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
        0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
        0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
        0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
        0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
        0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
        0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
        0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
        0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
        0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
        0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
        0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
        0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
        0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
        0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
        0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
    ];

    /// <summary>
    /// 计算 Modbus CRC-16 校验码
    /// </summary>
    /// <param name="data">要计算的数据字节数组</param>
    /// <returns>CRC校验码（低字节在前）</returns>
    public static ushort Compute(ReadOnlySpan<byte> data)
    {
        ushort crc = 0xFFFF;
        foreach (var b in data)
        {
            crc = (ushort)((crc >> 8) ^ CrcTable[(crc ^ b) & 0xFF]);
        }
        return crc;
    }

    /// <summary>
    /// 计算 CRC 并将结果添加到消息末尾（Modbus格式）
    /// </summary>
    /// <param name="data">原始数据</param>
    /// <returns>带 CRC 校验码的数据</returns>
    public static ReadOnlySpan<byte> Append(ReadOnlySpan<byte> data)
    {
        ushort crc = Compute(data);

        // 使用 stackalloc 避免堆分配（小数据时）
        if (data.Length <= 256)
        {
            Span<byte> result = stackalloc byte[data.Length + 2];
            data.CopyTo(result);
            result[data.Length] = (byte)(crc & 0xFF);
            result[data.Length + 1] = (byte)(crc >> 8);
            return result.ToArray();
        }
        else
        {
            // 大数据使用常规数组
            byte[] result = new byte[data.Length + 2];
            data.CopyTo(result);
            result[data.Length] = (byte)(crc & 0xFF);
            result[data.Length + 1] = (byte)(crc >> 8);
            return result;
        }
    }

    /// <summary>
    /// 计算 CRC 并将结果添加到消息末尾（Modbus格式）
    /// </summary>
    /// <param name="buffer">原始数据</param>
    /// <param name="len">数据长度</param>
    /// <returns>带 CRC 校验码的数据长度</returns>
    public static int Append(Memory<byte> buffer, int len)
    {
        ushort crc = Compute(buffer.Span);

        buffer.Span[len + 1] = (byte)(crc & 0xFF);
        buffer.Span[len + 2] = (byte)(crc >> 8);

        return len + 2;
    }

    /// <summary>
    /// 验证带 CRC 的数据是否有效
    /// </summary>
    /// <param name="dataWithCrc">包含 CRC 校验码的数据</param>
    /// <returns>验证结果</returns>
    public static bool Validate(ReadOnlySpan<byte> dataWithCrc)
    {
        if (dataWithCrc.Length < 2)
        {
            return false;
        }

        ushort receivedCrc = (ushort)(dataWithCrc[^1] << 8 | dataWithCrc[^2]);
        ushort calculatedCrc = Compute(dataWithCrc[..^2]);
        return receivedCrc == calculatedCrc;
    }
}
